1. Field of the Invention
The present invention relates to an improvement of output power of an X-ray generation tube, an X-ray generation device, and an X-ray imaging system.
2. Description of the Related Art
An X-ray generation tube is an X-ray source that is used for an X-ray generation device in an application of a medical diagnosis or a nondestructive inspection such as a foreign matter inspection. The X-ray generation tube includes an electron gun for emitting an electron beam, an anode for accelerating electrons, and a target for generating an X-ray by a collision of the electrons. The target is electrically connected to the anode.
It is known that, in order to obtain a predetermined analysis resolution, the X-ray generation device includes a grid electrode having an electrostatic lens action for a purpose of decreasing a focus diameter of the electron beam to be formed on the target.
Japanese Patent Application Laid-Open No. 2011-81930 describes an X-ray generation device including a lens electrode for focusing the electron beam, which is disposed between an electron emitting portion and the target.
On the other hand, when the X-ray generation tube outputs an X-ray, approximately 1% of kinetic energy of electrons included in the electron beam irradiating the target is used for the X-ray, and most of the input energy is converted into thermal energy, with the result that a temperature of the target rises.
The target of the X-ray generation tube is irradiated with the focused electron beam, and hence is apt to be thermally damaged at a region where current density distribution of the electron beam is largest.
Japanese Examined Utility Model Application Publication No. H04-3384 discloses a method of reducing thermal damage to the focusing center portion of the target by devising a cathode structure. Japanese Examined Utility Model Application Publication No. H04-3384 discloses the method in which a cathode filament is formed into a spiral shape, and an end of the filament is positioned in the center portion of the spiral filament, so as to decrease the temperature at the center portion of the electron beam, and to reduce the current density of the electron beam of emitted thermal electrons.